1. Field of the Invention:
The present invention relates to a sliding mode control method.
2. Description of the Prior Art:
Heretofore, a PID (proportional plus integral plus derivative) control process or a feedback control process employing an optimum regulator has generally been used to control a plurality of state quantities of an object to be controlled (e.g., the displacement of an object to be controlled and a rate of change of the displacement).
However, it is difficult for the conventional control process such as the PID control process or the like to converge the state quantities with sufficient quick response or stability against disturbances, characteristic changes of the object to be controlled, etc. For the optimum regulator, it is necessary to construct a model of the object to be controlled. As an error (a model error) between the model and the actual object to be controlled increases owing to dynamic characteristic changes of the object to be controlled, it is difficult to maintain sufficient quick response or stability for the convergence of the state quantities.
In view of the above conventional drawbacks, it has recently been practiced to control state quantities of an object to be controlled in a sliding mode control process according to the modern control technology.
The sliding mode control process is a feedback control process of variable structure. According to the sliding mode control process, a hyperplane (see FIG. 7 of the accompanying drawings) expressed by a linear function which has as its variables a plurality of state quantities of an object to be controlled is defined in advance, and the state quantities are converged onto the hyperplane under high-gain control. Furthermore, while the state quantities are being converged onto the hyperplane, the state quantities are converged toward a given balanced point (a point corresponding to target state quantities) by an equivalent control input.
The sliding mode control process has excellent characteristics in that once the state quantities are converged onto the hyperplane, the state quantities can stably be converged toward the balanced point without being substantially subject to effects of disturbances, etc. Therefore, it is possible to increase the quick response and stability of convergence of the state quantities against disturbances, etc.
For controlling the state quantities of the object to be controlled according to the sliding mode control process, it is desirable to increase the quick response and stability of convergence of the state quantities onto the hyperplane and also quick response and stability of convergence of the state quantities toward the balanced point on the hyperplane for the purpose of sufficiently maintaining the quick response and stability of convergence of the state quantities.
According to the inventor's understanding, if the state quantities in the sliding mode control process represent two values, then when the state quantities are already substantially converged onto the hyperplane, the stability of convergence of the state quantities toward the balanced point on the hyperplane is greater and the time required for the state quantities to converge toward the balanced point is shorter (the quick response is greater) as the gradient of the hyperplane is greater. When the state quantities are not still converged onto the hyperplane, the state quantities can be converged relatively quickly onto the hyperplane while being prevented from suffering oscillatory responses with respect to the hyperplane if the gradient of the hyperplane is smaller (the quick response can be increased).
According to the conventional sliding mode control process, therefore, it has been customary to construct a single hyperplane in a manner to keep in balance the quick response or stability of convergence of state quantities toward a balanced point on the hyperplane and the quick response or stability of convergence of the state quantities onto the hyperplane.
However, the sliding mode control process with the hyperplane thus constructed is unable to simultaneously increase both the quick response or stability of convergence of the state quantities toward the balanced point on the hyperplane and the quick response or stability of convergence of the state quantities onto the hyperplane. Consequently, it has been difficult to further increase the quick response or stability of convergence of state quantities of an object to be controlled toward target state quantities.